The present invention relates to resin transfer molding processes and preforms useful in those processes.
Resin transfer molding processes are used to make fiber reinforced composite materials. Layers of reinforcing material are loaded into a mold, and a thermosetting resin is injected into the mold and cured by ordinary and accepted procedures to provide a finished composite piece.
It is difficult and time consuming to load different layers of fiber into the mold. It was previously necessary to stitch the fibers together in order to provide net-shaped preforms.
More recently, a thermoplastic-like resin is sometimes used as a "tackifier" to stiffen the reinforcing material and hold it in place before the molding process begins. See Heck et al., U.S. Pat. No. 4,992,228 (Feb. 12, 1991) and Flonc et al., U.S. Pat. No. 5,080,851 (Jan. 14, 1992), which are incorporated herein by reference. ("Thermoplastic-like" means that the resin is a solid, thermosetting resin that exhibits thermoplastic properties, such as a glass-transition temperature and/or a melting point, below the temperature that cures the resin, so that the resin is thermmoformable.) In such a process, the individual fiber plies are sprinkled with a solid powder of the tackifier. The powder is heated to fuse it onto the surface of the substrate and then cooled to solidify. The different plies can be stacked together, heated to fuse the plies together, and then cooled leaving a preform. The preform can be placed into the mold and used in an ordinary resin transfer molding process thereafter.